Supported catalysts are often formed by combining materials with different functional characteristics into one composite material. Some of the most thoroughly explored and widely used catalysts are comprised of materials that serve three basic functions. Typical illustrative examples of these compositions are the familiar cobalt-molybdenum-alumina, and nickel-molybdenum-alumina catalysts. In these composites the component comprised of alumina typically ranges between 80–85% percent by weight, the molybdenum component ranges between 10–20% percent by weight, and the cobalt/nickel component typically ranges from 0–5% percent by weight of the final catalyst. Although many variations on materials and methods of production have been discovered, the descriptions of the roles of the materials that comprise these composites are generally the same.
The chemical and structural interactions of all these components and the feedstock are directly related to the activity and/or selectivity of the catalyst and therefore the description of their functions should be considered as conventionally accepted descriptions rather than conclusively definitive descriptions. In these composites, the small quantities of metals like cobalt, nickel and others may be referred to as promoters (P) the molybdenum component comprises the main catalytically active material (M) and the alumina component acts as the support structure (S). For the purposes of this application, these catalyst types will be categorized on the basis of the three generally accepted functions of their materials, and will be hereinafter referred to collectively as promoter-main-support “PMS” catalysts.
It should be understood that the selection of promoter materials and combinations of promoter materials (P) can vary widely, and numerous support materials other than, or in combination with, alumina have been identified as the preferred support materials (S) for specific applications. However, for the purposes of the current invention only group IVB, VB, and VIB elements of the periodic table of elements would be included in the (M) component of the catalyst. Typically (M) materials would be selected from sources of molybdenum and/or tungsten.
U.S. Pat. No. 2,486,361 to Nahin et al, is representative of many patents that teach the basic methods employed to produce PMS catalysts. Generally, processes for preparing catalysts of the PMS type employ methods for impregnation and/or co-precipitation of a support material. A significant number of modifications in impregnation techniques have been identified in literature. Typically, when preparing such a catalyst by impregnation, the support, in the form of powders, granules, or pellets of, for example, preformed gamma alumina, is either simultaneously, or sequentially impregnated with a solution of a suitable soluble salt, typically an aqueous solution of ammonium molybdate, and a soluble promoter salt, such as cobalt nitrate, or the like. Whereupon the carrier, having absorbed a portion of the solution, is dried and calcined at a temperature in the range of about 400 C. to about 700 C. to convert the adsorbed salt to the oxide of the metal or metals employed. The catalysts are then sulfided immediately prior to use.
In preparing a catalyst by co-precipitation the process usually embodies a simultaneous precipitation of the hydrated oxide of the support and typically the hydrated oxide or oxides of the desired catalytic agents from a solution containing appropriate amounts of the suitable soluble salt of the carrier type material and the metal or metals employed as catalytic agents. A common modification of this procedure consists of precipitating the hydrous oxides of the catalytic agent in the presence of a wet carrier gel.
Many other variations in co-precipitation have been described including those recorded in U.S. Pat. No 4,853,359 to Morrison et al., where we are taught methods for instigating a co-precipitation of support material and promoters with the simultaneous flocculation of dispersed sheets of exfoliated MoS2, WS2, and TaS2 to form a coating on the support. In addition to methods described in the 359 patent, some examples of variations in support materials that may be coated with exfoliated layered transition metal dichalcogenides, as well as methods for including various species between sheets that may be useful as promoters, are described in U.S. Pat. Nos. 6,143,359, 6,071,402, 5,932,372, 5,279,720, 4,996,108, and 4,822,590.
Many of the most dramatic increases in catalyst activity seen in recent years have been achieved, at least in part, by an increased ability to form porous support materials with high surface areas with defined pore sizes and pore volumes. It is believed that some of the benefit derived from these new supports is their ability to expose more of the active sites on the molybdenum or tungsten portion of the catalyst. Methods for converting exfoliated sheets of transition metal dichalcogenides into solid porous structures are described in U.S. Pat. No. 5,279,805 as well as the current inventors pending U.S. patent application Ser. No. 10/223,096.
It would be beneficial if effective means could be found for making improved PMS catalysts with a large selection of support materials and a large variety of promoters employed individually or in combination.
It would be difficult to conceive of a more obvious avenue to explore to improve activity in PMS catalysts, than that of increasing the content of active metals within a given volume of the catalyst. Methods for increasing active metal loadings in PMS catalysts have been reported. For example Simpson, in U.S. Pat. No. 4,111,795, describes a method for mulling catalyst materials and careful calcinations that apparently allows for up to 30% MoO3 by weight content. However, due to various limitations such as pore volume, pore size and surface area of the support, as well as other more imponderable factors, it has rarely been found possible to derive worthwhile benefit from increasing the metal loading of PMS catalysts beyond certain rather definite levels, usually between 18–20 weight percent MoO3 and 4–5 weight of promoter. As a consequence most commercial PMS type catalysts contain between about 12 and 16 weight percent MoO3 and about 2–5 weight percent of promoter. In commercially prepared PMS catalysts, activity reportedly begins to level off at about 16 weight percent MoO3, and at higher MoO3 levels, above about 22 weight percent, a very definite loss in activity is usually observed.
It would be beneficial if methods could be found to effectively load higher quantities of active metals per unit volume of catalyst.
The inventor has discovered that these and other benefits may be achieved when those skilled in the art combine known techniques for coating support materials with exfoliated sheets of layered transition metal dichalcogenides such as exfoliated MoS2, WS2 and the like, with the skilled application of known techniques for the filling the pores of the said coated supports as described by way of example in the methods below.